To start, prepare the dumpling filling. In a large bowl, combine the ground pork, chopped napa cabbage, green onions, ginger, garlic, mushrooms, baking soda (if using), sesame oil, mirin, oyster sauce, and soy sauce. Use your hands or a fork to mix everything together until well combined.

Next, set up your ramekins or mugs. Begin layering by adding a spoonful of the pork mixture to the bottom, followed by a wonton wrapper. Continue alternating layers of filling and wonton wrappers, pressing gently as you go, until the ramekin is nearly full.

Before adding the final layer, pour ⅓ cup of water into each ramekin. Top with two wonton wrappers to seal it in. Place the ramekins into a dutch oven or large pot with water halfway up the sides of the ramekins and put the lid on the pot.

Cover and steam for about 20 minutes, until the pork is fully cooked and the wonton wrappers are soft and tender. Carefully remove from the steamer, finish with chili oil and sliced green onions, and serve warm straight from the ramekin.